What is Adaptive
Optics?

Adaptive Optics refers to optical systems which adapt to
compensate for optical effects introduced by the medium between the object and
its image.

Under ideal circumstances, the resolution of an optical system is limited
by the diffraction of light waves. This so-called "diffraction limit" is generally
described by the following angle (in radians) calculated using the light's wavelength
and optical system's pupil diameter:

where the angle is given in radians. Thus, the fully-dilated human eye should
be able to separate objects as close as 0.3 arcmin in visible light, and the
Keck Telescope (10-m) should be able to resolve objects as close as 0.013 arcsec.

In practice, these limits are never achieved. Due to imperfections in the
cornea and lens of the eye, the practical limit to resolution only about 1 arcmin.
To turn the problem around, scientists wishing to study the retina of the eye
can only see details about 5 (?) microns in size. In astronomy, the turbulent
atmosphere blurs images to a size of 0.5 to 1 arcsec even at the best sites.

Adaptive optics (AO) provides a means of compensating for these effects,
leading to appreciably sharper images sometimes approaching the theoretical
diffraction limit. With sharper images comes an additional gain in contrast
-- for astronomy, where light levels are often very low, this means fainter
objects can be detected and studied.